The current digital cellular systems, such as GSM in Europe and DAMPS in the USA, are mainly based on large cells. The visions for mobile communications include a low-cost pocket telephone for free communication at home, in offices, city centres or suburbs, and in the countryside. For the services of the mobile telephone network to be functional in a wide area, large cells should be used in rural and suburban areas, and small cells in city centres and inside buildings in order to provide a sufficient capacity in areas in which the traffic demand is high. The service requirements are different in different environments. Speech transfer will be the most important service in most environments, but demand for data transfer service will be increasing, particularly for indoor communication.
As regards the modulation schemes, the system requirements vary in different environments. For instance, the conditions for signal propagation are different indoors and outdoors, as well as in small cells in which the aerials are located low and in large cells in which the aerials are high. In addition, the communication range varies from a couple of meters to tens of kilometers. In large cells the range is mainly limited by the peak power of the transmission which indicates that power amplifiers with higher efficiencies are needed. For this reason, constant envelope modulation schemes have traditionally been used in mobile communications because they allow the use of non-linear amplifiers which have a higher power efficiency than the linear amplifiers. A higher spectrum efficiency is achieved with the linear modulation schemes than with the constant envelope modulation schemes because the bandwidth efficiency of the linear modulation schemes is higher. The bandwidth efficiency refers to the bit rate of the channel divided by the channel bandwidth. The linear modulation schemes, however, require linear or linearized power amplifiers which in general are less power efficient than the non-linear amplifiers. Consequently, the linear modulation schemes are not optimal for large cells, and therefore, the constant envelope modulation schemes have traditionally been used in mobile telephone networks.
In small cells the output power levels are quite low, and the power consumption of a power amplifier is only a fraction of the total power consumption. For this reason, the use of linear modulation schemes is feasible in small cells, and would even be most desirable, because they offer higher capacity than the constant envelope modulation schemes.
U.S. Pat. No. 4,955,083 discloses a prior art method of cellular radio telephone communications wherein one modulation system is used in providing a signal and another modulations system is used for the transmission of actual information (i.e. speech).
Swiss patent CH632,362 discloses a cellular radio telephone communication system in which frequency and/or phase modulation is used. However, frequency and phase modulation cannot be considered as two different modulation systems as they are merely alternative ways of presenting the same modulation.
European patent EP 328836 discloses a cellular radio telephone communications system in which different modulation schemes are used in different directions i.e. to and from the mobile receiver.